1. Field of the Invention
The invention, in general, relates to a method of examining the authenticity of documents and, more particularly, to a method of examining the authenticity of documents provided with optico-diffractively effective markings.
2. The Prior Art
Complex and elaborate devices have hitherto been required for examining documents such as, for instance, bank notes, stock certificates and other securities provided with optico-diffractive indicia or markings, hereinafter sometimes referred to as holograms, for their authenticity. Such examining devices are not only complex; they also depend, for their proper functioning, upon great precision or close tolerances for aligning the documents relative to sensors. This, in turn and by necessity, mitigates against rapid examining and, accordingly, has prevented the incorporation of such equipment in high speed processing machines. More specifically, it has not been possible to examine the authenticity of bank notes provided with holograms in high-speed bank note counting machines as such machines typically operate at rates in excess of 1,500 note per minute.
German patent application 27 47 156 discloses a method and a testing apparatus for testing the authenticity of identity cards provided with holographically encrypted security indicia. The hologram is reproduced for performing a visual examination. Obviously, such a device is not suitable for rapid and efficient examinations independently of a person.
European patent specification 0,042,946 discloses an apparatus for generating scanning patterns for testing by a system including a laser, reflector and lens as well as a photo detector or sensor. This apparatus, too, is expensive and its adaptation for examining unsorted documents would be even more expensive, for it would require a multiple cascading arrangement of the testing system.
U.S. Pat. No. 4,255,652 teaches an electrically responsive indicia detecting apparatus in which at a first position an electrical charge is capacitively induced onto the detection indicia of a document moving to a second position. During such movement the induced charge leaks and the amount of leakage is measured at the second position to generate a signal for use in determining the manner of further processing of the document. Such a system is believed not to yield sufficiently reliable signals in view of the fact that the amount of charge leakage is a function of the quality of the indicia.